A facile synthesis of functionalized 7,8-diaza[5]helicenes through an oxidative ring-closure of 1,1'-binaphthalene- 2,2'-diamines (BINAMs)

Article abstract of DOI:10.3762/bjoc.11.2

A facile and moderately functional-group-tolerant synthetic method for the preparation of 7,8-diaza[5]helicenes has been developed. It comprises of an oxidative ring-closing process of 1,1'-binaphthalene-2,2'-diamine (BINAM) derivatives with a chlorinecontaining oxidant (t - BuOCl) in the presence of a base (2,6-lutidine). In addition the basic physicochemical properties of newly synthesized compounds have been investigated.

Full text of DOI:10.3762/bjoc.11.2

A facile synthesis of functionalized 7,8-diaza[5]helicenes
through an oxidative ring-closure of 1,1’-binaphthalene-
2,2’-diamines (BINAMs)
Youhei Takeda*1,2, Masato Okazaki2, Yoshiaki Maruoka2 and Satoshi Minakata*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2015, 11, 9–15.
1Frontier Research Base for Global Young Researchers, Graduate
School of Engineering, Osaka University, Yamadaoka 2-1, Suita,
Osaka 565-0871, Japan and 2Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Yamadaoka 2-1,
Suita, Osaka 565-0871, Japan
doi:10.3762/bjoc.11.2
Received: 23 October 2014
Accepted: 12 December 2014
Published: 05 January 2015
Email:
Associate Editor: B. Stoltz
Youhei Takeda* - takeda@chem.eng.osaka-u.ac.jp;
Satoshi Minakata* - minakata@chem.eng.osaka-u.ac.jp
© 2015 Takeda et al; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
aromatic amines; azahelicenes; cinnolines; halogen; oxidation
Abstract
A facile and moderately functional-group-tolerant synthetic method for the preparation of 7,8-diaza[5]helicenes has been devel-
oped. It comprises of an oxidative ring-closing process of 1,1’-binaphthalene-2,2’-diamine (BINAM) derivatives with a chlorine-
containing oxidant (t-BuOCl) in the presence of a base (2,6-lutidine). In addition the basic physicochemical properties of newly
synthesized compounds have been investigated.
Introduction
Helicenes, ortho-fused polycyclic aromatic compounds, have creating helicene-based novel functional molecules but also
been fascinating organic chemists over the last century since the uncovering missing pieces of unique aspects of helicenes. A
first synthesis of azahelicenes was reported by Meisenheimer in bibliographic survey about helicenes led us focus on diazahe-
1903 [1], not only because of their aesthetically attractive struc- licenes having a diazene (–N=N–) moiety, which is a member
tures, but also because of their unique properties arising from of the azahelicene family [5]: These compounds would be a
the helical chirality [2-11]. In fact, helicenes have been finding suitable model for probing the effects of the replacement of
more and more potential applications such as optoelectronic carbon atoms at the K-region of the carbohelicenes with
materials, asymmetric catalysts, and chemosensors [9]. There- nitrogen atoms on their structures and physicochemical prop-
fore, the development of synthetic methods for the preparation erties [12-14] and should serve as ligands to metal complexes
of helicenes which are difficult to access by conventional through the coordination at the diazene unit [15-17]. Taking
methodologies, would offer us many opportunities not only for 7,8-diaza[5]helicene as an example, conventional methodolo-
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Beilstein J. Org. Chem. 2015, 11, 9–15.
gies of preparing such compounds (Scheme 2) involve i) reduc- question, because of the involvement of the reduction process of
tive coupling of 2-nitronaphthalenes using strong reductants the oxides employing LiAlH4. Therefore, in conjunction with
like Zn dust [1,18] and PH3 [19], ii) reductive ring-closure of recent advances in preparative methods of functionalized
2,2’-dinitro-1,1’-binaphthalenes using various reductants such BINAMs [25,26], the development of oxidative ring-closing
as Na2S and LiAlH4 [20,21], and iii) the Scholl-type cycliza- methods that allow for one-step and functional-group-tolerant
tion of 2,2’-azonaphthalenes using a eutectic melt of AlCl3/ synthesis of 7,8-diaza[5]helicenes from BINAMs would be
NaCl [22]. However, these methods require harsh reaction desirable.
conditions such as the use of strong reductants or oxidants,
leading to low functional group compatibility. More impor- As a part of our project to develop oxidative transformations of
tantly, the reductive methods i) and ii) hardly avoid the produc- aromatic amines for the construction of diverse aza-containing
tion of N-oxide of diazahelicenes as well as azoxynaphthalenes π-conjugated functional molecules [27-30], we have recently
(Scheme 1), the N-oxides are quantitatively reduced by a strong discovered an iodine-containing oxidant-induced unusual oxida-
reductant to the corresponding diazahelicenes though. Another tive rearrangement of BINAMs leading exclusively to U-shaped
simple synthetic route to 7,8-diaza[5]helicenes would involve azaacenes (Scheme 2, the upper reaction) [31]. During a thor-
an oxidative ring-closure of 1,1’-binaphthalene-2,2’-diamines ough screening of oxidants for the rearrangement reaction, we
(BINAMs) (Scheme 1, iv). However, interestingly enough, this found out that the use of tert-butyl hypochlorite (t-BuOCl) as an
route remains to be explored. Corbett and Holt reported that the oxidant exclusively gave 7,8-diaza[5]helicene instead of the
oxidation of BINAM with NaBO3 only gave a trace amount of corresponding rearranged product in good yields. This prelimi-
the corresponding diazahelicene [23]. In this connection, nary finding prompted us to further explore the potential of this
Caronna and co-workers have recently reported a simple oxidative system as a facile and functional-group-tolerant syn-
protocol for synthesizing 7,8-diaza[5]helicene through oxi- thetic method of this class of diazahelicenes, since no N-oxide
dation of BINAM with mCPBA and following reduction of its byproducts are formed under this system and thereby any
oxides [24]. Although the method is high yielding in 2 steps, it reduction processes using strong reductants are not needed.
is not straightforward due to the formation of major amounts of Herein we present a facile, straightforward, and moderately
its N-oxide and N,N’-dioxide (Scheme 1). Furthermore, func- functional-group-tolerant synthesis of 7,8-diaza[5]helicenes
tional group compatibility of the method still remains an open (benzo[f]naphtho[2,1-c]cinnolines) bearing functional
Scheme 1: Synthetic routes to 7,8-diaza[5]helicene.
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Beilstein J. Org. Chem. 2015, 11, 9–15.
Scheme 2: Oxidant-controlled transformations of BINAMs.
substituents on the helical π-conjugated backbone through an gave low yields (ca. 20%) of 2a. Solvents also significantly
oxidative ring-closure of BINAM derivatives (Scheme 2, the affected the yields of 2a (Table 1, entries 7–11), and t-BuOH
lower reaction).
gave the best yield. In terms of the amounts of oxidant, the use
of 2 equivalents of t-BuOCl (0.4 mmol) gave a lower yield of
2a (54%, entry 12) with a moderate conversion of 1a, in com-
Results and Discussion
Initially, we began to identify the optimum reaction conditions parison with the case of using 4 equivalents of t-BuOCl (89%,
(oxidants, bases, and solvents) for the oxidative ring-closure of entry 1). In light of the reaction stoichiometry of the oxidative
BINAMs using 1a as the model substrate (Table 1, for the full process, 2 equivalents of HCl are supposed to be concomitantly
details of the screening of reaction conditions, see Supporting generated, which could have protonated diamine 1a leading to
Information File 1). As aforementioned in the Introduction inhibition of conversion of 1a. To trap the resulting HCl, the
section, we have recognized that the treatment of 1a with efficacy of base addition was investigated applying an ideal
4 equivalents of chlorine-containing oxidant (t-BuOCl) exclu- stoichiometry (2 equivalents) of t-BuOCl (Table 1, entries
sively gave diazahelicene 2a in a high yield (Table 1, entry 1) 13–17). The addition of an inorganic base (K2CO3) slightly im-
[31]. As related works to this project, we have reported that the proved the yield (Table 1, entry 13), while strong organic bases
unique iodine-containing reagent t-BuOI, which is readily like DABCO (pKa 8.93 in DMSO [32]) and DBU (pKa 23.9 in
generated in situ from t-BuOCl and NaI, serves as a powerful MeCN [33]) gave a lower yield (32%) and no product, respect-
oxidant for homo- and cross-dimerization of aromatic amines ively (Table 1, entries 14 and 15). As results, the use of the
leading to aromatic azo compounds [27,28,30]. However, the moderately weak organic base 2,6-lutidine (pKa 6.72 in water
employment of t-BuOI as an oxidant for this intramolecular [34]) successfully afforded 2a in high yield (Table 1, entry 16),
oxidative N=N forming reaction resulted in much lower yield of and a small excess amount (2.2 equiv) of t-BuOCl and 2,6-luti-
2a (Table 1, entry 2) while the skeletal rearrangement product dine gave diazahelicene 2a quantitatively (Table 1, entry 17).
was obtained as a main product (51% yield) [31], showing the When optically pure (R)-BINAM 1a (99% ee) was applied to
obvious superiority of t-BuOCl as an oxidant. Encouraged by the optimized reaction conditions, racemic 2a ([α]D22 = 0.0°,
these results, a variety of chlorine-containing oxidants were c 1.0, CHCl3) was obtained, losing the chiral information of 1a
tested to probe the influence of oxidants on the chemical yields through rapid racemization of optically active 2a or of the reac-
of 2a (Table 1, entries 3–6). While the N-monochlorinated tion intermediates.
reagents N-chlorosuccinimide (NCS, entry 3) and N-chloro-
phthalimide (NCPh, entry 5) did not produce 2a at all, di- and Having identified the optimized conditions for the oxidative
tri-chlorinated oxidants 1,3-dichloro-5,5-dimethylhydantoin ring-closure of 1a, we then turned our attention to applying
(DCH, entry 4) and trichloroisocyanuric acid (TCCA, entry 6) this method to the preparation of functionalized 7,8-
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Beilstein J. Org. Chem. 2015, 11, 9–15.
Table 1: The effect of reaction parameters on the oxidative ring-closure of 1a.
Entry
Oxidant (X)
Base (Y)
Solvent
Yield (%)a
Recovery of 1a (%)a
1
2
3
t-BuOCl (0.8)
t-BuOI (0.8)
NCS (0.8)
—
—
—
t-BuOH
t-BuOH
t-BuOH
89b
6c
0
0
0
63
4
5
—
—
t-BuOH
t-BuOH
24b
0
DCH (0.4)
0
95
NCPh (0.8)
6
—
t-BuOH
20
0
TCCA (0.27)
t-BuOCl (0.8)
t-BuOCl (0.8)
t-BuOCl (0.8)
t-BuOCl (0.8)
t-BuOCl (0.8)
t-BuOCl (0.4)
t-BuOCl (0.4)
t-BuOCl (0.4)
t-BuOCl (0.4)
t-BuOCl (0.4)
t-BuOCl (0.44)
7
—
EtOH
MeOH
THF
61
58
30
22
68
54
69
32
0
0
0
8
—
9
—
—
0
10
11
12
13
14
15
16
17
MeCN
toluene
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
t-BuOH
0
—
0
—
34
20
45
34
10
0
K2CO3 (0.4)
DABCO (0.4)
DBU (0.4)
2,6-lutidine (0.4)
2,6-lutidine (0.44)
90
97b
aDetermined by 1H NMR. bIsolated yields. cDibenzo[a,j]phenazine was obtained in 51% yield.
diaza[5]helicenes (Scheme 3). When diamine 1b bearing 3,3’- (Scheme 3). Notably, 3,3’-dibromo-substituted diamine 1c gave
dimethyl substituents was subjected to the optimized conditions a high yield of dibrominated diazahelicene 2c even in the
[t-BuOCl/2,6-lutidine (2.2 equiv) in t-BuOH at room tempera- absence of 2,6-lutidine, albeit in a longer reaction time
ture for 3 h], the desired helicene 2b was obtained in a low yield (Scheme 3). Such brominated helicene would be difficult to
(32%), even though electron-rich benzylic carbons labile to prepare by reductive methods using strong reductants like Zn
oxidative conditions are present. Modification studies (for the dust, due to competitive over-reduction of the bromo function-
details, see the Supporting Information File 1) revealed that the ality. It should be noted that the reaction efficiency of the oxida-
use of toluene as a solvent slightly improved the yield of 2b tive process was not affected by the steric congestion around the
12

Beilstein J. Org. Chem. 2015, 11, 9–15.
Scheme 3: Substrate scope of the oxidative ring-closure of BINAMs. Reaction conditions: 1 (0.1 mmol), t-BuOCl (0.22 mmol), 2,6-lutidine
(0.22 mmol), and solvent (10 mL) were stirred at the indicated temperature over the time shown below the respective product. Yields shown in paren-
theses indicate isolated yields. aRun with 4 equivalents of t-BuOCl in the absence of 2,6-lutidine.
aromatic amino moieties (Scheme 3, 2d). Diazahelicene 2e (3) leading to benzo[c]cinnoline (4) in a good yield, although
bearing two electron-withdrawing carboxylic ester groups was the modification of the reaction temperature was required
also successfully obtained in a high yield (84%) without (Scheme 4, for the detailed modification study of reaction
impairing the ester functionality (Scheme 3). Again, it is worth conditions, see Supporting Information File 1).
emphasizing that such a type of diazahelicene would be diffi-
cult to prepare by conventional reductive methods. Further-
more, a BINAM derivative installed with two alkyl substituents
at the 6,6’-position, 1f, was efficiently transformed to the
corresponding diazahelicene 2f in a good yield. Bromo func-
tionalities at the 6,6’-position of BINAM also survived the
oxidative conditions to give the corresponding diazahelicene 2g
(Scheme 3). Regarding the limitation of this oxidative method,
diamine substrates having strongly electron-donating
Scheme 4: Oxidative ring-closure of 3.
substituents (both the 3,3’-MeO and 7,7’-MeO) were not
successful, probably due to background side-reactions such as
chlorination on the aromatic rings and oxidative oligomeriza- A possible reaction pathway of the oxidative ring-closure of 1a
tion of BINAMs.
leading to 2a is illustrated in Scheme 5. Chlorination of an
amino group of 1a with t-BuOCl would generate N-monochlori-
Furthermore, this new oxidative method was found to be applic- nated BINAM (Int-A) and release t-BuOH. The N-chlorination
able in the ring-closing reaction of 1,1’-biphenyl-2,2’-diamine would induce umpolung reactivity of the amino moiety because
13

Beilstein J. Org. Chem. 2015, 11, 9–15.
Scheme 5: A possible reaction pathway of the oxidative ring-closure of 1a.
of the attachment of electronegative halogen species on the N
atom [27-30], and thereby an intramolecular nucleophilic unit
(–NH2) would attack the electrophilic N-center to form N–N
single bond (Int-B). The organic base (2,6-lutidine) would trap
HCl generated through this process. Another repetition of a
similar N-chlorination/HCl elimination cycle should give 2a via
Int-C.
Since some of the synthesized diazahelicenes 2 are new
members of 7,8-diaza[5]helicenes, basic physicochemical prop-
erties of 2a–2g were also investigated (for the detailed data, see
the Supporting Information File 1). Diluted dichloromethane
solutions of 2 (ca. 10−5 M) exhibited UV–vis absorption spectra
featuring weak absorptions ascribed to n–π* transitions in the
lower energy region (400–450 nm), an absorption (shoulder at
330–350 nm) and a strong absorption (300–330 nm) ascribed to
π–π* transitions, which are typical to aromatic ring-fused cinno-
Figure 1: UV–vis absorption spectra of 2a, 2b, 2d, and 2f.
line derivatives [13] (representative spectra are shown in suggesting promising electron-accepting abilities of diazahe-
Figure 1; for the full spectra, see Supporting Information licenes.
File 1). The introduction of two methyl (2b) and phenyl groups
(2d) at the 3,3’-position of BINAM skeleton resulted in a red- Conclusion
shift of the absorption spectra over the whole region (Figure 1). We have developed a facile and moderately functional-group-
In contrast, the introduction of 6,6’-n-Bu substituents (2f) tolerant method for the synthesis of 7,8-diaza[5]helicenes
caused a blue-shift of n–π* transitions (380–430 nm), while through an efficient oxidative ring-closure of BINAMs with the
π–π* transitions (300–340 nm) were red-shifted. On the other combination of t-BuOCl as an oxidant and 2,6-lutidine as a
hand, diluted dichloromethane solutions (10−5 M) of diazahe- base. This method has been applied to functionalized BINAMs
licenes 2 did not show photoluminescence (Φf < 0.02, for the except for highly electron-rich substrates to produce functional-
detailed values of quantum yields, see the Supporting Informa- ized 7,8-diaza[5]helicenes in a single-step from BINAMs.
tion File 1).
Furthermore, basic properties of the azahelicenes have also
been investigated.
Due to the presence of the electronegative diazene (–N=N–)
moiety, diazahelicenes 2 are expected to possess stabilized
LUMO energies. The LUMO energies, which were estimated
from cyclic voltammetry (CV) experiments [ELUMO =
–(4.8 + redE1/2) eV] using diluted CH2Cl2 solutions (10−4 M) of
2, range from –2.92 to –3.13 eV (for the full cyclic voltammo-
grams and the LUMO energies, see Supporting Information
File 1). These LUMO energy levels fall in a similar range of the
U-shaped azaacenes that our group previously reported [31] and
well-known electron-transporting materials Alq3 [35],
Supporting Information
Supporting Information File 1
Experimental procedures, characterization data, copies of
NMR charts, UV–vis spectra, cyclic voltammograms, and
TGA profiles.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-11-2-S1.pdf]
14

Beilstein J. Org. Chem. 2015, 11, 9–15.
21.Corbett, J. F.; Holt, P. F. J. Chem. Soc. 1960, 3646–3653.
Acknowledgements
doi:10.1039/jr9600003646
This research was partly supported by the research Grant from
The Murata Science Foundation, the Sasakawa Scientific
Research Grant from The Japan Science Society, and the
Research Encouragement Grants from The Asahi Glass Founda-
tion (to Y.T.). Also, one of the authors (Y.T.) would like to
acknowledge all the warm support from the Frontier Research
Base for Global Young Researchers, Osaka University, on the
Program of MEXT.
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